I can understand why a lot of Superman purists wanted to see the writers craft a different storyline where he didn't have to kill Zod.

But why? Why take the easy route? I know Superman has killed before, but the vast majority of writers over the years always put him in a situation where he finds some way to outwit the enemy without resorting to killing them.

I thought it was actually a bit of a ballsy move by Snyder to have Superman being forced into an no-win situation where ultimately, someone was going to die. And he had to make the difficult choice of whether that was going to be an innocent human(s), or Zod.

Is it against his moral code? Yes - you can see how he struggled with it.
Is it against our almost messianic image of Superman? Yes, to a certain extent.

But that messianic image has only been formed because decades of writers decided that when push came to shove, it was much easier to have Zod suddenly be dispatched to the phantom zone, or have Superman come up with a Plan B at the end, or for a third party to intervene so that he would never have to kill whoever he was fighting.

In this situation in MOS, there was no other way out, no other options, and no Plan B for Superman. A film isn't like the comics, where characters appear, disappear, die and come back to life many times over in the course of 5, 10 , 20 years. A film has to have a certain finality. He had to make a horrible decision, and I applaud the writers for actually going through with it for once.